Eco-friendly pb-free free cutting steel with excellent machinability and hot workability

ABSTRACT

There is provided a Pb-free free cutting steel is formed of 0.03 to 0.30 wt % of carbon (C), 0.01 to 0.30 wt % of silicon (Si), 0.2 to 2.0 wt % of manganese (Mn), 0.02 to 0.10 wt % of phosphorus, 0.06 to 0.45 wt % of sulfur (S), 0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn), 0.001 to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N), 0.002 to 0.025 wt % of total oxygen (T[O]), and residual Fe, and unavoidable impurities, wherein S, Bi, S, B, and N satisfy a certain relationship. The steel has excellent machinability no less than conventional free cutting steel including Pb, while being eco-friendly. Also, the steel has excellent hot ductility capable of reducing occurrence of defects on a surface, thereby improving hot rolling workability.

TECHNICAL FIELD

The present invention relates to eco-friendly Pb-free free cutting steelused as a material of precision oil pressure parts of an automobile,parts for office automation equipment, and parts for home appliances andmore particularly, to eco-friendly free cutting steel with excellentmachinability by using not only elements for improving themachinability, capable of replacing Pb harmful to environments or ahuman body, but also oxides with low melting point, formed on a steelwire rod by precision deoxidization. In addition, the present inventionrelates to eco-friendly free cutting steel where defects on a surface,such as corner cracks, do not occur while hot rolling, due to excellenthot ductility thereof.

BACKGROUND ART

Free cutting steel is generally used as a material for precisioncomponents, which has excellent machinability. The excellentmachinability of the free cutting steel is due to metallic ornonmetallic inclusions present in the free cutting steel. When cuttingsteel products by using a tool, nonmetallic inclusions such as MnS actas a stress concentration element at a portion where a tip of the toolis in contact with steel products in such a way that generation of voidsand growth of cracks at an interface between the inclusions and a matrixare easily made and power required in cutting is reduced.

Also, metallic inclusions such as Pb melt at a relatively lowertemperature than cutting heat and act as a lubricant at an interfacebetween a chip and a cutting tool, thereby restraining abrasions of thecutting tool and reducing cutting force.

Accordingly, to improve machinability of steel products, elementscapable of forming the metallic or nonmetallic inclusions are added. Asconventionally used nonmetallic inclusions, there is MnS. Particularly,MnS formed in a spherical shape mixed with oxides provides mostexcellent machinability.

On the other hand, metallic inclusions are generally called asmachinability improving elements. Pb is a representative machinabilityimproving element. Since Pb has low solubility with iron, it is easy toexist in free cutting steel as metallic inclusions. Also, due to anappropriately low melting point of 327.5° C., Pb is capable of beingeasily melted by heat generated in a cutting tip.

Accordingly, since Pb thoroughly has properties required for improvingmachinability, up to now, free cutting steel containing Pb is classifiedas most representative free cutting steel and has been put to practicaluse as most suitable steel products for cutting.

However, the free cutting steel containing Pb may generate lead vapor ina process of recycling cutting operations. Since Pb present in steelproducts is harmful to a human body, it has been required from long agoto replace the steel having Pb.

As steel products developed to replace the free cutting steel containingPb, there is free cutting steel having Bi. Since Bi is a low meltingpoint metal and has low solubility for iron, Bi is very advantageous toimprove machinability.

However, since the melting point of Bi is 290° C., which is lower thanthat of Pb by 120° C., Bi is easier to be melted. Since having surfacetension lower than Pb, Bi has high wettability. Such properties causeembrittlement of grain boundaries of steel products.

Accordingly, due to a decrease in hot ductility caused by theembrittlement of grain boundaries, the free cutting steel having Bi hasnotably deteriorated hot workability. Also, the free cutting steel hasmachinability not good as that of the free cutting steel containing Pb,there still exist various problems to replace the steel having Pb by thesteel having Bi.

However, there are also problems in free cutting steel containing Pb.Particularly, as CNC machine tools have been rapidly provided, highspeed cutting and automation are realized. There occurs a phenomenonwhere a certain element of a cutting tool, such as tungsten (W) that ismost important element of a tungsten carbide, diffuses to a chip at highspeed by heat with a temperature of 1000° C. or more in the high speedcutting. Due to such diffusion of the element such as W, the cuttingtool may be rapidly worn.

Particularly, since the free cutting steel containing Pb is not capableof effectively preventing abrasion of tools, due to thermal diffusion,it is required to develop free cutting steel having excellentmachinability in an aspect of high speed cutting.

DISCLOSURE OF INVENTION Technical Problem

An aspect of the present invention provides eco-friendly Pb-free freecutting steel having eco-friendly properties by adding Bi and Sn capableof replacing elements harmful to environments and human bodies, such asPb, to steel products, providing excellent machinability by formingcomposite oxides having a low melting point, capable of restrainingabrasions of tools, and having excellent hot workability by addingelements such as Mn and B by optimal ratio.

Technical Solution

According to an aspect of the present invention, there is providedPb-free free cutting steel is formed of 0.03 to 0.30 wt % of carbon (C),0.01 to 0.30wt % of silicon (Si), 0.2 to 2.0 wt % of manganese (Mn),0.02 to 0.10 wt % of phosphorus (P), 0.06 to 0.45 wt % of sulfur (S),0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn), 0.001to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N), 0.002to 0.025 wt % of total oxygen (T[0]), and residual Fe, and unavoidableimpurities, wherein Sn, Bi, S, B, and N satisfy one or more relationsselected from a group consisting of following Relational Expressions 1to 3,

$\begin{matrix}{{\frac{\left( {{Bi} + {Sn} + S} \right)}{Mn} \geq 0.4},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 1} \\{{\frac{{Mn}^{3}}{S} \geq 4.6},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 2} \\{and} & \; \\{\frac{B}{N} \geq {2.0.}} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 3}\end{matrix}$

According to another aspect of the present invention, there is providedPb-free free cutting steel is formed of 0.03 to 0.30 wt % of C, 0.01 to0.30wt % of Si, 0.2 to 2.0 wt % of Mn, 0.02 to 0.10 wt % of P, 0.06 to0.45 wt % of S, 0.04 to 0.20 wt % of Bi, 0.04 to 0.20 wt % of Sn, 0.001to 0.015 wt % of B, 0.001 to 0.010 wt % of N, 0.002 to 0.025 wt % ofT[0], and residual Fe, and unavoidable impurities, wherein the steelincludes one of MnO—SiO—Al₂O₃-based oxides, CaO—SiO—Al₂O₃-based oxides,and composite oxides with a low melting point, which is a mixture of theMnO—SiO₂—Al₂O₃-based oxides and the CaO—SiO—Al₂O₃-based oxides.

Advantageous Effects

According to an exemplary embodiment of the present invention,eco-friendly Pb-free free cutting steel having excellent machinability,no less than free cutting steel containing Pb. Also, due to excellenthot ductility, occurrence of defects on a surface while hot rolling iscapable of being reduced, thereby improving hot workability.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, the present invention provides Pb-free free cuttingsteel showing excellent properties not only at low speed cutting butalso at high speed cutting by controlling an element system, arelationship between elements, a number of low melting point compositeoxides, respectively or cooperatively.

Hereinafter, an element system forming the Pb-free free cutting steelaccording to the present invention will be described in detail.

Carbon (C): 0.03 to 0.30 wt %

To provide surface roughness and mechanical properties, C should beadded by 0.03 wt % or more, and more particularly, to 0.05 wt % or more.However, when adding C more than 0.30 wt %, machinability becomesdeteriorated due to an increase of hard pearlite structures.

Silicon (Si): 0.01 to 0.30 wt %

Si acts as a deoxidizer and generates SiO₂. To form low melting pointcomposite oxides capable of reducing abrasions of a tool due to thermaldiffusion while cutting at high speed, Si may be added to 0.01 wt % ormore, and more particularly, to 0.05 wt % or more. However, when addingsilicon more than 0.30 wt %, high melting point inclusions or exclusiveSiO₂ inclusions area formed, thereby notably increasing speed ofabrasions of tools.

Manganese (Mn): 0.2 to 2.0 wt %

Mn forms MnS inclusions, which prevent red shortness caused by S. Mn maybe added to 0.2 wt % or more. However, when adding Mn more than 2.0 wt%, ferrites are solid-solution strengthened, which reducesmachinability. Mn acts as a deoxidizer, forms MnO, and acts as a nucleusof MnS inclusions.

Phosphorus (P) 0.02 to 0.10 wt %

P is segregated into boundaries and improves machinability. P may beadded to 0.02 wt % or more. However, to provide mechanical propertiesand cold workability, P may be added 0.10 or less.

Sulfur (S): 0.06 to 0.45 wt %

S forms MnS inclusions, which restrains generation of a built-up edge toreduce abrasions of a cutting tool and improve surface roughness of aworkpiece in a cutting process. For this, S may be added to 0.06 wt % ormore. However, when an amount of S becomes great, it is easy to generateFeS with a low melting point, which decreases hot ductility and make hotrolling difficult. Therefore, the amount of S should be 0.45 wt % orless.

Bismuth (Bi): 0.04 to 0.20 wt %

Bi independently exists as metal inclusions or attached to MnSinclusions when adding to steel products. Bi is easily melted by heatwhile cutting, which improves cutting properties, reduces frictionalforce by acting as a lubricating film between a chip and a cutting tooland restrains abrasions of the cutting tool. When a content of Bi isless than 0.04 wt %, an effect of machinability is decreased. On theother hand, when the content of Bi is more than 0.20 wt %, and moreparticularly, 0.16 wt %, casting and rolling properties are not good.The content of Bi may be limited within a range from 0.04 to 0.20 wt %.

Tin (Sn): 0.04 to 0.20 wt %

Sn may act similarly to Pb. That is, Sn may act identically to liquidmetal embrittlement that is a mechanism of Pb, improving machinabilityof steel. In detail, this phenomenon is shown since Sn moves ferritegrain boundaries and is segregated, and decreases binding energy ofgrain boundaries, thereby allowing the grain boundaries to be easilyweakened. Accordingly, to obtain an effect of improving machinabilitydue to Sn, Sn may be added to 0.04 wt % or more. However, when a contentof Sn is more than 0.20 wt %, and more particularly, 0.16 wt %, it maybe harmful to casting and rolling properties. Therefore, the content ofSn may be limited to be in a range from 0.04 to 0.20 wt %.

Boron (B): 0.001 to 0.015 wt %

B segregated into austenite boundaries improves hot ductility bystrengthening grain boundaries. Also, it has been known since earlytimes that steel containing graphite has excellent machinability. When Breacts to nitrogen in steel and a B nitride BN having a grain structureand physical properties similar to the graphite is generated, there maybe an effect of improving machinability identical to the steelcontaining graphite. When a content of B is less than 0.001 wt %, aneffect of adding B is very small. Accordingly, B should be added to0.001 wt % or more. On the other hand, when the content of boron is morethan 0.015 wt %, there is no additionally increased effect and grainboundary strength is decreased due to precipitation of the boronnitride, thereby deteriorating hot workability. The content of B may belimited to be within a range from 0.001 to 0.015 wt %.

Nitrogen (N): 0.001 to 0.010 wt %

N should be added to 0.001 wt % or more to form BN together with boron.However, when a content of N is more than 0.010 wt %, an amount ofeffective boron segregated into austenite grain boundaries is reduced,thereby decreasing boundary strengthening effect.

Total Oxygen (T[0]): 0.002 to 0.025 wt %

It is required to add oxygen (0) of 0.002 wt % or more to prevent adecrease of machinability, due to MnS inclusion elongation while hotrolling. However, a content of T[0] should be 0.025 wt % or less toprovide plastic deformability of the MnS inclusions while cutting.

Aluminum (Al) and Calcium (Ca): 10 ppm or less, respectively

Al and Ca are required to form low melting point composite oxides formedin steel. However, it is not required to intentionally add. An amountnaturally included in slag is enough. Al and Ca may be generally presentby 10 ppm or less.

Among the described elements, Bi, Sn, S, Mn, and B may provide excellentmachinability and hot workability by satisfying following relationalexpressions. Hereinafter, the relational expressions with respect to Bi,Sn, S, Mn, and B will be described in detail.

Relational expression with respect to Sn, Bi, S, and Mn is as follows.

$\begin{matrix}{\frac{\left( {{Bi} + {Sn} + S} \right)}{Mn} \geq 0.4} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 1}\end{matrix}$

wherein each symbol for element indicates weight percent (wt %) of theelement, the same as above.

In addition to the restriction on contents of the elements, to providePb-free fee cutting steel having excellent machinability according to anexemplary embodiment of the present invention, Relational Expression 1should be satisfied. That is, Sn and Bi causes improvement ofmachinability by liquid metal embrittlement in steel products, asmetallic inclusions and S improves machinability by forming MnS.

Relational expression with respect to Mn and S is as follows.

$\begin{matrix}{\frac{{Mn}^{3}}{S} \geq 4.6} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 2}\end{matrix}$

In addition to the restriction on the contents of the elements, toprovide the Pb-free free cutting steel having excellent hot ductility,it is required that a relation between Mn and S satisfies RelationalExpression 2. Relational Expression 2 indicates that a content Mn isrequired to a degree to be bonded to S and restrain hot embrittlementdue to S.

Relational Expression with respect to B and N is as follows.

$\begin{matrix}{\frac{B}{N} \geq 2.0} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 3}\end{matrix}$

To provide the Pb-free free cutting steel having excellent hotductility, B and N should satisfy Relational Expression 3. That is,though N is present, there is required just an amount of N capable ofstrengthening austenite grain boundaries by B segregated into grainboundaries.

Though satisfying one of Relational Expressions 1 to 3, an effectthereof is shown. When satisfying two or more of Relational Expressions1 to 3 at the same time, there is shown a notable effect. Accordingly,when satisfying one or more of Relational Expressions 1 to 3, it may beconsidered as being included in the scope of the present invention.

On the other hand, the Pb-free free cutting steel of the presentinvention includes low melting point inclusions by Mn, Si, Ca, and Al.Hereinafter, the low melting point inclusions will be described indetail.

In the element system of the present invention, oxidization of Mn, Si,Ca, and Al occurs, thereby various low melting point composite oxidesare formed. To form the inclusions, Mn, Si, Ca, and Al may beadditionally added. However, an amount of Ca and Al basically present insteel is enough to form the inclusions. In the present invention, theoxides may be present in the form of MnO—SiO₂—Al₂O₃-based orCaO—SiO₂—Al₂O₃-based.

The MnO—SiO₂—Al₂O₃-based oxides may be formed of 20 to 65 wt % of MnO,25 to 60 wt % of SiO , and 0 to 30 wt % of Al₂O₃. TheCaO—SiO—Al₂O₃-based oxides may be formed of 10 to 55 wt % of CaO, 35 to65 wt % of SiO₂, and 0 to 25 wt % of Al₂O₃.

Also, one of the low melting point composite oxides such as theMnO—SiO₂—Al₂O₃-based oxides and the CaO—SiO —Al₂O₃-based oxides may bepresent five or more per 5 g of a steel wire rod. When there are lessthan five inclusions, machinability is decreased.

Hereinafter, embodiments of the present invention will be described indetail.

Embodiment

Turning test and high temperature tensile test were performed oninventive steels and comparative steels having compositions shown inTables 1, 2, and 3 to investigate machinability, hot ductility thereof.Composite oxides were analyzed by extraction & separation of nonmetallicinclusion in steel by electrolysis in AA solution under ultrasonic wave(ESAA).

TABLE 1 C Si Mn P S B Bi Sn T[O] N Inventive 0.079 0.067 1.155 0.0530.304 0.0095 0.07 0.08 0.0080 0.0048 Steel 1 Inventive 0.073 0.060 1.1510.067 0.328 0.0092 0.13 0.14 0.0120 0.0032 Steel 2 Inventive 0.102 0.0801.235 0.058 0.350 0.0070 0.09 0.11 0.0153 0.0015 Steel 3 Inventive 0.0440.030 1.570 0.059 0.380 0.0100 0.18 0.17 0.0140 0.0023 Steel 4 Inventive0.038 0.010 1.250 0.061 0.310 0.0074 0.13 0.09 0.0170 0.0035 Steel 5Inventive 0.104 0.086 1.380 0.058 0.360 0.0120 0.08 0.11 0.0160 0.0027Steel 6 Comparative 0.080 0.138 1.449 0.050 0.376 0.0073 0.10 0.100.0130 0.0058 Steel 1 Comparative 0.070 0.004 1.162 0.076 0.344 — 0.070.11 0.0201 0.0043 Steel 2 Comparative 0.290 0.285 1.020 0.030 0.2100.0081 — 0.05 0.0110 0.0040 Steel 3 Comparative 0.070 0.002 1.10 0.0800.298 — Pb: — 0.0120 — Steel 4 0.3

In Table 1, Inventive Steels 1 to 6 and Comparative steel 1 satisfy theelement system of the present invention. On the other hand, ComparativeSteels 2 and 3 are different in B and Bi and Comparative Steel 4indicates conventional free cutting steel containing Pb.

TABLE 2 (Bi + Sn + S)/Mn Mn³/S B/N Inventive Steel 1 0.4 5.07 2.0Inventive Steel 2 0.5 4.65 2.9 Inventive Steel 3 0.4 5.38 4.7 InventiveSteel 4 0.5 10.18 4.4 Inventive Steel 5 0.4 6.30 2.1 Inventive Steel 60.4 7.30 4.4 Comparative Steel 1 0.4 8.09 1.3 Comparative Steel 2 0.54.56 0.0 Comparative Steel 3 0.3 5.05 2.0

In Table 2, it may be known that Comparative Steels 1 and 2 are out ofan appropriate range of B/N and Comparative Steel 3 is out of anappropriate range of (Bi+Sn+S)/Mn. Comparative Steel 4 is not shownbecause Comparative Steel 4 is free cutting steel containing Pb.

TABLE 3 MnO—SiO₂—Al₂O₃ - CaO—SiO₂—Al₂O₃ - based based Number ofcomposite oxides composite oxides inclusions (per SiO₂ Al₂O₃ CaO SiO₂Al₂O₃ 5 g of steel wire MnO (%) (%) (%) (%) (%) (%) rod) Inventive 30 5515 40 35 25 10 Steel 1 Inventive 45 35 20 35 45 20 7 Steel 2 Inventive25 50 25 15 65 20 6 Steel 3 Inventive 50 25 25 45 20 15 9 Steel 4Inventive 50 40 10 30 55 15 12 Steel 5 Inventive 35 40 25 45 50 5 8Steel 6 Comparative 60 30 10 40 35 25 5 Steel 1 Comparative 80 10 10 2530 45 2 Steel 2 Comparative 40 40 20 30 55 15 6 Steel 3 *ESSA:extraction & separation of nonmetallic inclusion in steel byelectrolysis in AA solution under ultrasonic wave

Also, in Table 3, it may be known that a number of inclusions includedin Comparative Steel 2 are less than a reference value. Also,Comparative Steel 4 that is the free cutting steel containing Pb isexcluded.

With respect to Inventive Steels and Comparative Steels, to checkwhether Inventive Steels are capable of replacing the free cutting steelcontaining Pb by testing machinability of Inventive Steels,machinability tests were performed as follows. In the machinabilitytest, a workpiece that was a bar with a diameter of 25 mm was turned bya CNC lathe without cutting oil. A transfer speed was 0.3 mm/rev, acutting depth was 0.5 mm, and a cutting speed was 150 m/min. To check adegree of abrasions of a tool, after turning test for the same time, aflank wear width (VB) of the tool was measured and compared. A result oftool abrasions caused by the turning operation was shown in Table 4.

TABLE 4 Tool flank wear width according to cutting time (mm) Cutting for10 Cutting for 20 Cutting for 30 minutes minutes minutes Inventive Steel1 0.12 0.21 0.30 Inventive Steel 2 0.07 0.14 0.20 Inventive Steel 3 0.090.18 0.28 Inventive Steel 4 0.07 0.12 0.18 Inventive Steel 5 0.08 0.160.24 Inventive Steel 6 0.10 0.19 0.28 Comparative Steel 1 0.08 0.15 0.25Comparative Steel 2 0.15 0.23 0.32 Comparative Steel 3 0.20 0.34 0.40Comparative Steel 4 0.16 0.28 0.34

As shown in Table 4, as a result of measuring the degree of toolabrasions via a cutting test, when comparing eco-friendly free cuttingsteels according to the present invention, which were Inventive Steels 1to 6 with conventional free cutting steel containing Pb, which wasComparative Steel 4, the eco-friendly free cutting steels showed veryexcellent tool wear resistant properties. In the case of ComparativeSteel 2, since low melting point oxides were not formed, machinabilitythereof was less excellent than that of Inventive Steels. Also, in thecase of Comparative Steel 3, tools were most rapidly abraded.

To perform a hot ductility test, Inventive Steels and Comparative Steelswere heated at a temperature of 1250° C. that was a reheatingtemperature and maintained for one minute. After that, a tension testwas performed. After the tests, a reduction of area (RA) of specimen wasmeasured and shown in Table 5.

TABLE 5 Reduction of Reduction of Reduction of Reduction of area at areaat area at area at 900° C. 1000° C. 1100° C. 1200° C. Inventive 70 72 8489 Steel 1 Inventive 69 78 82 92 Steel 2 Inventive 76 82 85 91 Steel 3Inventive 62 65 72 80 Steel 4 Inventive 73 78 83 93 Steel 5 Inventive 7775 80 91 Steel 6 Comparative 50 60 62 83 Steel 1 Comparative 49 57 60 76Steel 2 Comparative 49 57 60 76 Steel 3 Comparative 77 81 88 81 Steel 4

As shown in Tables 1 and 2, in the case of Inventive Steels 1 to 6, avalue of Mn³/S was 4.6 or more, red shortness due to forming of lowmelting point FeS was restrained, and also, a value of B/N was 2.0 ormore, an effect of strengthening austenite grain boundaries was capablebeing obtained. Accordingly, it was possible to obtain hot ductilitywith a reduction of area of 70% when performing high temperature tensiletests at 900° C. or more. Therefore, a possibility of occurrence ofdefects on a surface such as corner cracks was very low.

On the other hand, as in the case of Comparative Steel 1, when a valueof Mn³/S was 4.6 or more but a value of B/N was less than 2.0, since Bin steel was generally precipitated and it was impossible to strengthengrain boundaries enough, a reduction of area less than 60% at atemperature of 900° C. was shown. Also, as in the case of ComparativeSteel 2, when a value of Mn³/S was less than 4.6 and a value of B/N wasless than 2.0, hot ductility was shown as lower than that of ComparativeSteel 1.

As described above, according to an exemplary embodiment of the presentinvention, there is provided eco-friendly Pb-free free cutting steelcapable of providing excellent machinability by restraining toolabrasion that may be shown in cutting processes at a speed regardless ofhigh or low speed by controlling contents of B, Sn, Mn, S, and N atappropriate relational expressions and forming low melting pointcomposite oxides, and having excellent hot workability by addingelements such as Mn and B at optimum ratios.

1. A Pb-free free cutting steel comprising 0.03 to 0.30 wt % of carbon(C), 0.01 to 0.30 wt % of silicon (Si), 0.2 to 2.0 wt % of manganese(Mn), 0.02 to 0.10 wt % of phosphorus (P), 0.06 to 0.45 wt % of sulfur(S), 0.04 to 0.20 wt % of bismuth (Bi), 0.04 to 0.20 wt % of tin (Sn),0.001 to 0.015 wt % of boron (B), 0.001 to 0.010 wt % of nitrogen (N),0.002 to 0.025 wt % of total oxygen (T[0]), and residual Fe, andunavoidable impurities, wherein S, Bi, S, B, and N satisfy one or morerelationships selected from a group consisting of following RelationalExpressions 1 to 3, $\begin{matrix}{{\frac{\left( {{Bi} + {Sn} + S} \right)}{Mn} \geq 0.4},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 1} \\{{\frac{{Mn}^{3}}{S} \geq 4.6},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 2} \\{and} & \; \\{\frac{B}{N} \geq {2.0.}} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 3}\end{matrix}$
 2. A Pb-free free cutting steel comprising 0.03 to 0.30 wt% of C, 0.01 to 0.30 wt % of Si, 0.2 to 2.0 wt % of Mn, 0.02 to 0.10 wt% of P, 0.06 to 0.45 wt % of S, 0.04 to 0.20 wt % of Bi, 0.04 to 0.20 wt% of Sn, 0.001 to 0.015 wt % of B, 0.001 to 0.010 wt % of N, 0.002 to0.025 wt % of T[0], and residual Fe, and unavoidable impurities, whereinthe steel comprises one of MnO—SiO₂—Al₂O₃-based oxide,CaO—SiO₂—Al₂O₃-based oxide, and composite oxides with a low meltingpoint, which is a mixture of the MnO—SiO₂—Al₂O₃-based oxide and theCaO—SiO₂—Al₂O₃-based oxide.
 3. The steel of claim 2, wherein theMnO—SiO₂—Al₂O₃-based oxide is formed of 20 to 65 wt % of MnO, 25 to 60wt % of Si02, and 0 to 30 wt % of Al₂O₃.
 4. The steel of claim 2,wherein the CaO—SiO₂—Al₂O₃-based oxide is formed of 10 to 55 wt % ofCaO, 35 to 65 wt % of SiO₂, and 0 to 25 wt % of Al₂O₃.
 5. The steel ofclaim 2, wherein there are five or more of the composite oxides with alow melting point in 5 g of a steel wire rod.
 6. The steel of claim 2,wherein the S, Bi, S, B, and N satisfy one or more relationshipsselected from a group consisting of following Relational Expressions 1to 3, $\begin{matrix}{{\frac{\left( {{Bi} + {Sn} + S} \right)}{Mn} \geq 0.4},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 1} \\{{\frac{{Mn}^{3}}{S} \geq 4.6},} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 2} \\{and} & \; \\{\frac{B}{N} \geq {2.0.}} & {{Relational}\mspace{14mu} {Expression}\mspace{14mu} 3}\end{matrix}$